The giant boring machine had just passed its 1,000-foot mark, about one-eighth of the route from Sodo to South Lake Union, and was progressing at an above-average pace before this latest hitch.

The rotary cutting head can chew through concrete, and includes steel discs that are designed to gouge and crack boulders. It cannot grind through steel.

The machine is 60 feet below surface, between South Jackson Street and South Main Street. The soil is considered to be firm glacial till but immediately above it is weak fill soil, including wood debris and sediments from the 1898 Denny Regrade, when Denny Hill ended up in Elliott Bay.

One option might be to deploy specially trained tunnel workers to crawl out to the cutter face with pneumatic drills or jackhammers and break up the clog. Tunnel contractors keep professional divers on call for such work, which is not underwater but is underground in greater-than-atmospheric pressures. Bertha has three special hatches to help divers reach the face.

The machine can retreat 18 inches, creating room for such work. But because tunnel crews would need to pressurize the air immediately in front of the machine,
the air might push or burst through the weak soil to the surface, said Yerkan.

Or maybe engineers will think up some other way.

If the object can be broken into fragments less than 3 feet in diameter, those would fit through cutter openings and could be moved out the back along a conveyor belt.

Chris Dixon, project manager for Seattle Tunnel Partners (STP), did not reply to three requests for comment Monday.

“STP has not made a decision on how they’re going to move forward yet,” Yerkan said. “They’re talking to their experts; we have been talking to ours.”

This is the fourth delay since boring started July 30. A labor dispute, a clog in the conveyor screw, some hardened grout in a soil-wetting system and now the mystery object have cost the project about two months — though Dixon has said previously it’s possible to make up time by adding overnight or weekend shifts. A small sinkhole opened last month but didn’t slow the work.

There are no reports of damage to the drill face, where eroded steel discs were recently replaced during routine maintenance.

“The machine is running well; it’s functioning,” Yerkan said.

The cutting face, at 57 feet, 4 inches, is the widest in the world. It’s equipped with steel cutting discs to scour and crack boulders.

The tunnel route was intensively sampled by soil engineers from Shannon & Wilson long before the project started, but apparently their narrow test shafts didn’t strike this object. Yerkan said that STP used “ground penetrating” radar before drilling but didn’t find any unusual objects in that area.

Hypersensitive measuring devices are posted every 50 feet near the waterfront to detect ground movement. But they can’t spot a buried object, Yerkan said.

By early 2014 the machine is supposed to dive under the Alaskan Way Viaduct, causing a temporary highway closure and potential risks of vibration to old brick buildings nearby.

Asked why project officials waited two days to disclose the problem, Yerkan offered two theories: that leaders were gone during the weekend, so Monday was the logical time for an update; and that the tunnel team may have wanted to come up with some progress or strategy to offer, before reporting publicly there was a problem.